1. Field of the Invention
The present invention relates to a reflector which provides uniform brightness and whiteness over an extensive range, a manufacturing method for the same, and a reflection type liquid crystal display device that employs the reflector.
2. Description of the Related Art
In recent years, a reflection type liquid crystal display device has been extensively used as the display unit of a handy type computer or the like. The reflection type liquid crystal display device is equipped with a reflector for reflecting the light which has entered the display surface thereof to provide display. In the past, reflectors having mirror surfaces or reflectors having random pits and projections on the surfaces thereof have been used.
Among the aforesaid conventional reflectors, a conventional reflector 260 equipped with a surface with random pits and projections is illustrated in FIG. 16. This reflector is produced by heating a polyester film 261 which is, for example, 300 to 500 xcexcm thick, to form an uneven surface 261a having projections of a few xcexcm high, then further forming a reflection film 262 composed of aluminum, silver, etc. on the uneven surface 261a by employing such a process as vapor deposition.
As illustrated in FIG. 17, in the conventional reflection type liquid crystal display device employing this type of reflector 260, transparent electrode layers 253 and 254 are provided on the opposed surfaces of a pair of glass substrates 251 and 252, respectively. Further, oriented films 255 and 256 of liquid crystal are respectively provided on the transparent electrode layers 253 and 254 and a liquid crystal layer 257 is disposed between the oriented films 255 and 256. A first polarizing plate 258 and a second polarizing plate 259 are provided on the outer sides the glass substrates 251 and 252, respectively, the reflector 260 being mounted on the outer side of the second polarizing plate 259 so that the surface thereof facing the reflection film 262 is oriented toward the second polarizing plate 259.
In the reflection type liquid crystal display device 250 having the constitution described above, the light which has entered the first polarizing plate 258 is linearly polarized through the polarizing plate 258, and the polarized light passes through the liquid crystal layer 257 to be elliptically polarized. The elliptically polarized light is then linearly polarized again through the second polarizing plate 259, and the linearly polarized light is reflected by the reflector 260 and it passes through the second polarizing plate 259 and the liquid crystal layer 257 again before it exits from the first polarizing plate 258.
The reflector and the reflection type liquid crystal display device have the following reflection characteristics.
For instance, as illustrated in FIG. 16, the incidence angle of an incident light J from a point light source disposed on the reflection film 262 is set to a constant incidence angle of 30 degrees with respect to the normal line relative to the surface of the reflection film 262, and the reflectivity is measured when a reflection angle xcex8 of reflected light K is changed from zero degree to 60 degrees. The measurement results have revealed that the reflectivity reaches almost a lowest level at a reflection angle of 20 degrees or less and 40 degrees or more at right and left, the peak of the reflectivity being observed at a reflection angle of 30 degrees. This trend has been found to be applicable to the measurements of an entire liquid crystal display device equipped with the reflector as well as to the reflector used alone. It has been discovered that the reflectivity reaches a peak at the reflection angle of 30 degrees, and it drops to almost zero percent at reflection angles of 23 degrees or less and 37 degrees or more.
In general, a reflector having a mirror surface exhibits a reflection characteristic in which extremely higher reflectivity is observed at a particular reflection angle in relation to an incidence angle than that in a reflector having random pits and projections on a surface thereof; it is characterized, however, by an extremely limited range of reflection angles at which high reflectivity is obtained, that is, it provides a limited range of visual field angles.
As described above, the conventional reflector with a reflecting surface equipped with random pits and projections has poor reflection efficiency with consequent low reflectivity as a whole, thus failing to fully meet the needs for a reflector that enables efficient reflection of incident light over a wider range of reflection angle. Accordingly, the reflection type liquid crystal display device employing this type of reflector has been posing a problem in that the visual field angles are limited to a range of about 25 degrees to about 35 degrees and that the brightness of the display surface is not satisfactory. There has been another problem: a reflector is required to provide whiteness as well as brightness; however, this type of conventional reflector is unsatisfactory in the whiteness of the reflecting surface because the light having different wavelengths cannot be reflected evenly in good balance. Further, the reflection characteristics including reflection angle and the intensity of reflected light of this type of reflector are automatically decided by the pits and projections formed at random; hence, they have not been controlled by optical design.
To solve the aforesaid problems, a reflector provided with many stripe grooves extending linearly on a surface thereof has been proposed. This reflector, however, has a limited range of reflection angle although it provides desired brightness at the reflection angles in a certain range in the direction perpendicular to the stripe grooves and it exhibits low reflectivity and an extremely limited range of reflection angles in the directions other than the direction perpendicular to the stripe grooves. Hence, the application of this type of reflector to a liquid crystal display device has not solved the foregoing problem of the limited range of visual field angles and insufficient brightness and whiteness of the display surface especially in the direction parallel to the stripe grooves.
Accordingly, the present invention has been made with a view toward solving the problems described above, and it is an object thereof to provide: a reflector capable of providing high reflection efficiency over a wide range of angles; a method for manufacturing the reflector; and a reflection type liquid crystal display device employing the reflector for the display surface thereof to provide a wider range of visual field angles and a higher level of brightness in any directions.
To these ends, according to one aspect of the present invention, there is provided a reflector wherein: many pits are formed in succession on a surface thereof, the inner surface of each of the pits being formed as a partial sphere; the pits are formed to have depths ranging from 0.1 xcexcm to 3 xcexcm at random, adjacent pits being disposed at random at pitches ranging from 5 xcexcm to 50 xcexcm; and the tilt angles of the inner surfaces of the pits are set within a range of xe2x88x9218 degrees to +18 degrees.
In the reflector in accordance with the present invention, many pits having the inner surfaces shaped as partial spheres are formed on a surface thereof, and the parameters thereof such as the depths of the pits and the pitches between adjoining pits are set to the ranges given above. By so doing, the tilt angles of the inner surfaces of the pits, namely, the tilt angles in a minute unit area, which are considered to govern the reflection angles of reflected light show a fixed distribution pattern in a certain range of angles. Further, since the inner surfaces of the pits are shaped like partial spheres, such a fixed distribution pattern of tilt angles can be accomplished in any directions rather than only in a particular direction in the reflector. Hence, the reflector in accordance with the present invention is able to provide uniformly high reflection efficiency in all directions so as to enable light having diverse wavelengths to be reflected in good balance. This makes it possible to realize a reflector that provides a higher level of brightness and whiteness than that of the conventional reflectors regardless of the direction from which it is observed.
The term xe2x80x9cthe depth of the pitsxe2x80x9d refers to the distance from the surface of the reflector to the bottoms of the pits; and the term xe2x80x9cthe pitches between adjacent pitsxe2x80x9d refers to the center-to-center distances of the pits which are circular in a top plan view. Further, the term xe2x80x9cthe tilt angles of the inner surfaces of the pitsxe2x80x9d means the following: when a minute range having a width of 0.5 xcexcm is taken at an arbitrary spot of the inner surface of a pit 4 as illustrated in FIG. 8, the term refers to angle xcex8 in relation to the horizontal surface of the slope in the minute range. The sign of angle xcex8 is defined so that angle xcex8 of the slope, for example, at right in FIG. 8 with respect to the normal line drawn onto the surface of the reflector carries the positive sign, while that of the slope at left carries the negative sign.
Preferably, the depths of the pits are set to 0.1 xcexcm to 3 xcexcm, the pitches between adjoining pits are set to 5 xcexcm to 50 xcexcm, and the tilt angles of the inner surfaces of the pits are set in the range of xe2x88x9218 degrees to +18 degrees as described above.
In particular, it is important to set the tilt angles so that they are distributed in the range of xe2x88x9218 degrees to +18 degrees and to set the pitches of the adjacent pits at random with respect to all directions in plane. This is important because, regularity in the pitches between adjoining pits would lead to an inconvenience in that an interference color of light develops, causing the reflected light to be colored. If the distribution of the tilt angles of the inner surfaces of the pits exceeds the range of xe2x88x9218 degrees to +18 degrees, then the angle of divergence of reflected light increases excessively and the reflection intensity drops. This makes it impossible to obtain a reflector with satisfactory brightness because the angle of divergence of the reflected light exceeds 36 degrees in the air and the peak of the reflection intensity in the liquid crystal display device goes down with a resultant increased total reflection loss.
If the depths of the pits exceed 3 xcexcm, it would be impossible to cover the apexes of the projections with a flattening film in a subsequent process for flattening the pits, failing to accomplish desired flatness.
If the pitches between adjacent pits are below 5 xcexcm, it would mainly cause the following problems: it would take much more time for processing because there are restrictions on the fabrication of a matrix for forming the reflector; the configuration that provides desired reflection characteristics cannot be obtained; and interference light emerges. For a practical use, if a diamond indenting tool that has a diameter of 30 xcexcm to 100 xcexcm usable for making the matrix for forming the reflector is employed, it is preferable to set the pitch of adjoining pits to 5 xcexcm to 50 xcexcm.
More preferably, the reflector in accordance with the present invention has the pits ranging from 0.6 xcexcm to 1.2 xcexcm in depth, the tilt angles of the inner surfaces being distributed in the range of xe2x88x928 degrees to +8 degrees, and the pitches between adjacent pits ranging from 26.5 xcexcm to 33.5 xcexcm.
When the tilt angles of the inner surfaces of the pits are distributed within the range from xe2x88x928 degrees to +8 degrees, the angle of divergence of the reflected light increases, making it possible to implement a brighter reflector.
Setting the depth of the pits to 0.6 xcexcm or more controls excessive regular reflection, and setting them to 1.2 xcexcm or less facilitates further flattening in the subsequent process. Likewise, setting the pitches between adjacent pits to 26.5 xcexcm or more permits a shorter time required for fabricating the matrix for forming the reflector, and setting it 33.5 xcexcm or less makes the configurations of the pits visually unrecognizable, thus leading to a higher quality of the reflector.
In the manufacturing method for the reflector in accordance with the present invention, a transfer mold having a mold surface, which has the reversed pattern of the pits and projections of the mold surface of a matrix for forming the reflector of a predetermined shape, is formed and the mold surface of the transfer mold is transferred to the surface of the base material for the reflector, then a reflection film is formed on the pits and projections of the surface of the base material for the reflector to complete the reflector.
Specifically, according the aforesaid method, the mold surface of the matrix for forming the reflector is directly transferred to the surface of the reflector via the transfer mold so as to form many pits having the inner surfaces shaped as partial spheres on a surface thereof. By so doing, the tilt angles of the inner surfaces of the pits, namely, the tilt angles in a minute unit area, which are considered to govern the reflection angles of reflected light, show a fixed distribution pattern within a certain range of angles. Further, since the inner surfaces of the pits are shaped like partial spheres, such a fixed distribution pattern of tilt angles can be accomplished in any directions rather than only in a particular direction in the reflector. Hence, the reflector in accordance with the present invention is able to provide uniformly high reflection efficiency in all directions so as to enable light having diverse wavelengths to be reflected in good balance. This makes it possible to realize a reflector that provides a higher level of brightness and whiteness than that of the conventional reflectors regardless of the direction from which it is observed.
The matrix for forming the reflector in accordance with the present invention is constituted by many pits which have the inner surfaces thereof shaped like partial spheres and which are formed in succession on the surface of the base material for the matrix, the pits ranging from 0.6 xcexcm to 1.2 xcexcm in depth, the tilt angles of the inner surfaces of the pits being distributed in the range of xe2x88x928 degrees to +8 degrees, and the pitches between adjacent pits ranging from 26.5 xcexcm to 33.5 xcexcm.
Preferably the matrix for forming the reflector has pits that range from 0.6 xcexcm to 1.2 xcexcm in depth, the tilt angles of the inner surfaces of thereof being distributed in the range of xe2x88x928 degrees to +8 degrees, and the pitches between adjacent pits ranging from 26.5 xcexcm to 33.5 xcexcm.
The term xe2x80x9cthe depths of the pitsxe2x80x9d refers to the distance from the surface of the base material for the matrix to the bottoms of the pits; and the term xe2x80x9cthe pitches between adjacent pitsxe2x80x9d refers to the center-to-center distances of the pits which are circular in a top plan view. Further, the term xe2x80x9cthe tilt angles of the inner surfaces of the pitsxe2x80x9d means the following: when a minute range having a width of 0.5 xcexcm is taken at an arbitrary spot of the inner surface of a pit 104 as illustrated in FIG. 15, the term refers to angle xcex8 in relation to the horizontal surface of the slope in the minute range. The sign of angle xcex8 is defined so that angle xcex8 of the slope, for example, at right in FIG. 15 with respect to the normal line drawn onto the surface of the reflector carries the positive sign, while that of the slope at left carries the negative sign.
According to the manufacturing method of the matrix for forming the reflector in accordance with the present invention, an indenting tool having a spherical distal end is pressed against the surface of the base material for the matrix repeatedly by changing the position of the indenting tool on the surface of the base material for the matrix. By so doing, many pits having their inner surfaces shaped like partial spheres are formed in succession to complete the matrix for forming the reflector.
Specifically, according to the manufacturing method of the matrix for forming the reflector in accordance with the present invention, the matrix for molding the reflector provided with a number of pits having their inner surfaces shaped like partial spheres is produced by pressing an indenting tool with a spherical distal end against the base material for the matrix by using a rolling apparatus. The indenting tool used for this purpose is repeatedly pressed a great number of times against the surface of the base material for the matrix composed of a metal material such as brass, stainless steel, or tool steel having relatively high hardness; therefore, it is desirable to employ an indenting tool composed of diamond or other material having high hardness. The rolling apparatus repeats pressing while changing the position of the indenting tool on the surface of the base material for the matrix to form many pits in succession. The requirement in this case is the relative movement of the base material for the matrix and the indenting tool within the horizontal plane; hence, either the base material for the matrix or the indenting tool may be moved.
To form the foregoing pits, the distance of the vertical stroke of the indenting tool of the rolling apparatus, the horizontal moving distance of the base material for the matrix, the diameter of the distal end of the indenting tool, etc. should be adjusted so as to set the depths of the pits to be formed within the range of 0.1 xcexcm to 3 xcexcm at random, the pitch of adjoining pits within the range of 5 xcexcm to 50 xcexcm at random, and the tilt angles of the inner surfaces of the pits to be distributed within the range of xe2x88x9218 to +18 degrees.
More preferably, the depths of the pits are set within the range of 0.6 xcexcm to 1.2 xcexcm, the tilt angles of the inner surfaces of the pits are distributed within the range of xe2x88x928 to +8 degrees, the pitch of adjoining pits is set within the range of 26.5 xcexcm to 33.5 xcexcm, and the matrix base material provided with the pits is used as the matrix for forming the reflector.
The reflection type liquid crystal display device in accordance with the present invention is equipped with the foregoing reflector, namely, the reflector that has many pits formed in succession on a surface thereof, the inner surface of each of the pits being formed as a partial sphere; wherein the pits are formed to have depths ranging from 0.1 xcexcm to 3 xcexcm, adjacent pits being disposed at pitches ranging from 5 xcexcm to 50 xcexcm; and the tilt angles of the inner surfaces of the pits are set within a range of xe2x88x9218 degrees to +18 degrees. The reflector may be of an external installation type mounted on the outer side of a liquid crystal cell or a built-in type mounted on the inner surface of the substrate constituting the liquid crystal cell.
The reflection type liquid crystal display device in accordance with the invention has the reflector that is able to provide high reflection efficiency in all directions so as to enable light having diverse wavelengths to be reflected in good balance. This makes it possible to implement a wider visual angle and a brighter display surface than those of the conventional reflection type liquid crystal display devices.